Suppression of equatorial spread <i>F</i> by sporadic <i>E</i>

Abstract

We have examined quantitatively the influence a low‐latitude, premidnight sporadic E layer might have on the daily and hourly development of equatorial spread F (ESF). In particular, we calculated changes in the flux tube–integrated Pedersen conductivity as it affects the growth rate of the Rayleigh‐Taylor instability, which governs the initial development of ESF. We find that the growth rate is lowered by an order of magnitude with a density of 1 × 10 6 cm −3 in a slab from 115 to 120 km. Since sporadic E layers observed after dusk do not regularly reach these values, they are not a likely source of the daily variability in ESF. However, even a mild enhancement in the postsunset E region could lead to a significant suppression of ESF if it also inhibits the upward plasma drift of the prereversal enhancement, a key variable in the growth rate of the equatorial spread F instability. Thus, consistent with the nature of an instability, the second‐order effect (suppressed upward drift) is more important than the first‐order cause (reduced F region to E region conductivity) of inhibited ESF onset.